Poor Kodaka, I think Yukimura is confusing the hell out of his sexual identity.
Fanservice aside, I think Kobato's "transformation" was a nice step forward for her. It's appropriate that her "true form" as a vampire is a lot closer to what you'd imagine a girl like her might normally wear. Though I have to wonder if they stopped buying her normal clothes at some point, and that's why she defaulted to her t-shirt from when she was 10.
The funny thing is that I don't think Kobato had any intention of flashing her brother. I bet she truly intended to perform a wicked-cool transformation sequence for him, and was legitimately disappointed that she couldn't pull it off. Er... pull it on?
What do you think, Onii-chan?
I will be so disappointed if they don't keep up this apparent running gag of everyone completely forgetting that Maria even exists by the 1/3 mark of the episode. She makes an adorable Zoidberg.